Author Topic: "Refractors are best for planetary viewing"  (Read 397 times)

canreosenbi

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Re: "Refractors are best for planetary viewing"
« Reply #15 on: January 30, 2018, 04:47:31 AM »
A bit of advice is not to judge planetary performance on one night of observing. There are nights where the refractor of a smaller aperture will beat the larger newt or SCT. And there are nights where it will be the opposite.

Best view of Saturn I had was through a 30 inch F3.3 dob. Aperture can do some wonderful things.

Other considerations are cool down and collimation. Those soft views in the SCT/newt could have been those or the seeing conditions (or all 3).

I think a reasonable telescope would be a 5 inch refractor for comparison to an 8 inch SCT. There will be times the 8 inch wins, but in general the quick cool down will be helpful. And in average to poor seeing I think the refractor would win.

So these decisions often depend on where you live and the local conditions.

Michael Consumers

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Re: "Refractors are best for planetary viewing"
« Reply #16 on: January 31, 2018, 07:23:37 AM »
Cool down is probably a consideration in the Boston area. But chromatic aberration is another consideration as you know from your 4 inch achromat.

Even a 6 inch SCT is worth considering. My C6 provides very nice images of planets. During the winter I use my refractors most of the time for the quick cool down.

meisporbiopop

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Re: "Refractors are best for planetary viewing"
« Reply #17 on: January 31, 2018, 09:38:12 AM »
Quote
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my best view of Jupiter was a 6" refractor
my best view of Saturn was a 10" Mac
my best view (and only) of Pluto was a 22" dob
my best view of comets was with binoculars
my best view of meteors and Northern Lights was naked eye

as stated above-it is complicated

edj

I like this answer. Different tools for different jobs. 

There isn't one perfect scope that does it all, which is why people wind up with two or three (or 10) telescopes.

I'm a refractor guy. Love my 6" f/8 achro. But it is a big scope on a big mount, and sometimes I'm not up for all that. Which is why I grabbed the MUCH smaller 6" SCT.

Between a C8 and a 6" achro, that's a tough call. In most fights aperture wins. The C8 sure would be more compact and easier to mount, and it has no CA. CA is a personal thing though... some are bothered by it, some aren't.
I think that it comes down to a combination of cost, quality, portability and observing goals

edj
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Waka Belcher

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Re: "Refractors are best for planetary viewing"
« Reply #18 on: February 02, 2018, 01:53:50 PM »
I haven't used a 6" achromat but I would expect it to have considerablechromatic aberrationthat would hurt contrast at the powers needed for planetary viewing. So for planetary I would still choose the 8" SCT. (For a good APO things would be more mixed.) I've had very good planetary views throughmy SCTover the past 21 years. For DSO's the 8" will out perform the 6". I haven't had seriousthermal issues with the 8". In my experience it has been sufficient to set it outside pointed at the sky 30 mins to an hour before I am planning to observe.

Planets respond to aperture assuming the optics are good. I'll take theresolution and light gatheringof an 8" over a 6" for detecting faint moons: Titania, Oberon, Mimas, Enceladus, Triton, etc. And Pluto is stillmore likely to be detected in an 8" SCT than in a 6"...although I did manage to detect Pluto this summer from a very dark site with a 6" off-axis aperture mask on my big Dob, it was less difficult with the 8" mask and with 8" SCT. It was easy with the 10" and 20".

But a 10" or 20" Dob still providemore detailed planetaryviews. The difference with a 10" is more subtle.

adrajacte

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Re: "Refractors are best for planetary viewing"
« Reply #19 on: February 02, 2018, 03:01:17 PM »
I think it can be debated whether a 6 inch refractor and an 8 inch SCT would be better for DSOs. Considering many factors (central obstruction, reflectivity of mirrors, etc) I suspect it would be pretty close. Especially true if the refractor was an APO or ED. Planetary I agree the chromatic aberration would be a bit annoying, but still useable. If planetary was the goal I'd use the SCT.

Now, based on the cost of an APO or the weight/mounting of a 6 inch refractor, I'd probably pick the 8 in SCT.

Some refractors are heavier than others and that needs to be considered. My Vixen 130mm is pretty light weight for a 5 inch refractor. Which allows me to use it on a pretty light weight EQ mount similar to an 8 inch SCT requirement.

subliliva

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Re: "Refractors are best for planetary viewing"
« Reply #20 on: February 02, 2018, 08:59:07 PM »
I'll just add my $0.02 and agree that it's complicated. As in rugby though, a good big 'un will beat a good little 'un just about every time. As an example, on a night of good seeing, <a id="anonymous_element_52" hovercard-ref="member" hovercard-id="12430" data-ipb="noparse">Cotts[/url]' 12" Teeter Dob ran rings around my TOA150. That evening left me less than gruntled.

On the 6" achromat vs C8 ... If they're stable and they're both collimated, I'd bet (a little) on the C8. Chromatic aberration degrades image clarity and I find it especially problematic on planets. If you added a Wratten #3 or #8, it might be a lot closer but then you would have a yellow moon.

stunfalriave

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Re: "Refractors are best for planetary viewing"
« Reply #21 on: February 03, 2018, 12:29:18 PM »
Well I'd say the C8 has a definite lead in the voting now. "Refractors are best for views of planets" - myth busted? Or is the jury still out? I think it would be fair to include an APO if it hits the same price point, like the ES 127mm APO.

My guess is that a C8 or a 7" MCT would still be better than a 5" APO. But the refractor's ace in the hole seems to be contrast - is it possible that while more detail may *theoretically* be there in a Cassegrain, the lower sharpness means it's not as visible as the details in a refractor?

tioraigenroi

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Re: "Refractors are best for planetary viewing"
« Reply #22 on: February 04, 2018, 09:41:06 AM »
Quote
I think it can be debated whether a 6 inch refractor and an 8 inch SCT would be better for DSOs. Considering many factors (central obstruction, reflectivity of mirrors, etc) I suspect it would be pretty close. Especially true if the refractor was an APO or ED. Planetary I agree the chromatic aberration would be a bit annoying, but still useable. If planetary was the goal I'd use the SCT.

I don't see it being close on DSO's.8" vs 6"is an extra 0.62 magnitude. Even allowing some for reduced transmissionand central obstruction it is still going to be about 0.4 magnitudes of difference and without the chromaticaberration problem. And you still retain the extra resolving power of the full 8" vs. 6" aperture. I would anticipate it being quite noticeable on globulars in how many begin to resolve, how deeply they resolve, andhow bright they appear overall. And for galaxiesthe closer you get to the limits of visibility the more apparent it will be there as well.

The area where the 6" will havedistinct advantage is in the low power wide field views.

Tyler Cox

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Re: "Refractors are best for planetary viewing"
« Reply #23 on: February 09, 2018, 05:28:35 AM »
APO/ED refractors are the best for views of planets if the aperture is the same (or close) is my opinion. Unless you had a very optimized newtonian of a longer focal length.

An issue here we are not addressing is optical performance of the lens or mirror. Not all telescopes are of the same quality. High end refractors tend to have better optics than your typical run of the mill C8. And of course you pay for it as well.

So if you compared my 130mm ED refractor to a C6/C5 I would think the refractor would be equal to the C6, and win on contrast. But for your C8 (assuming good seeing, cooled down, and collimated), the C8 may win. We are talking different apertures though and that has to be considered.

MCTs are good alternatives for planets, but cool down in the larger sizes can be an issue.

Stephen Artman

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Re: "Refractors are best for planetary viewing"
« Reply #24 on: February 09, 2018, 06:56:32 AM »
Quote
Quote

I think it can be debated whether a 6 inch refractor and an 8 inch SCT would be better for DSOs. Considering many factors (central obstruction, reflectivity of mirrors, etc) I suspect it would be pretty close. Especially true if the refractor was an APO or ED. Planetary I agree the chromatic aberration would be a bit annoying, but still useable. If planetary was the goal I'd use the SCT.

I don't see it being close on DSO's.8" vs 6"is an extra 0.62 magnitude. Even allowing some for reduced transmissionand central obstruction it is still going to be about 0.4 magnitudes of difference and without the chromaticaberration problem. And you still retain the extra resolving power of the full 8" vs. 6" aperture. I would anticipate it being quite noticeable on globulars in how many begin to resolve, how deeply they resolve, andhow bright they appear overall. And for galaxiesthe closer you get to the limits of visibility the more apparent it will be there as well.

The area where the 6" will havedistinct advantage is in the low power wide field views.
I think if you compared the scopes directly it would be closer than you may think.

The below Cloudynights article talks to this...it's an interesting discussion at the least:

http://www.cloudynig...mparision-r1901

EDIT: I should note that mounting a 6 inch refractor is more demanding than a 8 inch SCT. And if you were to buy a 6 inch APO/ED, you are laying down some serious cash. A 6 inch achromat will suffer from chromatic aberration pretty severe (still nice scopes though). So to the original poster who is likely looking between the C8 and the 6 inch F8 achromat, I'd pick the C8 for sure. Though I have looked through the 6 inch F8 many times and it's quite impressive on DSOs.

Best way to be sure is to look through both side by side and see what you like more.